Equine Digestive Tract Week 2 Lecture 4 Clair Thunes, PhD Animal Science 126 Equine Nutrition Goals Know the main anatomical sections of the hindgut, their functions and purpose Sites of potential impactions Understand degradation of feed in the equine hindgut and the end products Know what nutrients are absorbed in the different sections of the equine hindgut The Hindgut All grazing herbivores have an enlargement in their GI tract to allow for microbial fermentation. Ferment structural carbs and produce VFA s and lactate > half the dry weight of feces is bacteria The bacterial cells of the horse number >10 times the tissue cells of the body (Frape, D. 2010) Mammals can not enzymatically degrade Cellulose Hemicellulose Pectin Fructo and galacto oligosaccharides Lignin Bacteria can do all but lignin 1
Cecum. Animal Science 126 Equine Nutrition The Cecum Digesta leaving the SI relatively uniform in nature Fibrous feed material Undigested feed starch and protein Microbes Cellular lining Intestinal Secretions Large amount of bicarb released at the distal end of ilium Enters the cecum through ileocecal orifice Blind sac 1 meter long in adult Holds 25-35 liters Strong values in close proximity control entry from ileum and exit to R.Ventral colon through cecocoli orifice Cecum "The Horse - Its Treatment In Health And Disease", by J. Wortley Axe. 1905 2
Haustra http://cal.vet.upenn.edu/p rojects/grossanat/largeme nu/habdcecum.htm Colon. Animal Science 126 Equine Nutrition Colon Digesta leave cecum and enter the right ventral colon. The great colon or ascending colon has 4 parts; Right ventral (sternal flexure) Left ventral (pelvic flexure) Left Dorsal (diaphragmatic flexure) Right dorsal About 3-4m long in adult 2x capacity of cecum 3
Sternal flexure Diaphragmatic flexure Pelvic flexure "The Horse - Its Treatment In Health And Disease", by J. Wortley Axe. 1905 http://www.admani.com/allianceequine/equinedigestion.htm Colon Changes of function and microbial population by region Flexures likely foci of impactions Diameter greatest in RDC ~ 50cm Narrows to 7-10cm at entrance to small colon Small colon is ~ 3m long Rectum ~ 30cm long 4
Movement Through The Hindgut Animal Science 126 Equine Nutrition Contractions. Circular and longitudinal muscles in walls of SI and LI. Necessary for; Peristaltic contractions towards anus for movement of digesta Mixing of digestive juices Bathing intestinal walls with digestive products Movements stop with abdominal pain Gas build up Contractions - Cecum. Cecum contracts in a ring ~ 12-15cm from cecocolic junction/oriface Traps some digesta at base of cecum and forces some into ventral colon. According to Hintz 20% of digesta pass through cecum per hour Compares to 2-8% per hour for the rumen 5
Contractions - Colon. Bursts of contractions some originate away from mouth, some orally, some isolated and have no specific direction. Therefore non-rhythmic haustral kneading and stronger rhythmic propulsive and retropulsive contractions cause; mixing, promote fermentation, Promote absorption Movement of residue towards rectum Electrical pacemaker in pelvic flexure Left ventral colon major site of impactions Contractions cont. More contractions during eating. Cecal gas moves separately to digesta and moves rapidly through colon. Considerable mixing within sections but little retrograde flow due to various barriers; Ileocecal value Cecoventral colonic valve Ventrodorsal colonic (pelvic) flexure Dorsal small colonic junction Resistance to flow increases with each barrier Large food particles move more slowly Contractions cont. Particles of 2cm may remain for a week In ponies fed grain diet 10% voided after 24h, 50% after 36h and 95% after 65h (Frape, D., 2010) MRT in 18 month old horses fed hay and concentrate was 42.7 and 33.8h for the solid and liquid phases (Chiara et al., 2003) For a hay based diet it was 21-40h MRT decreasing as intake increased (Miraglia et al., 2033) 6
Contractions cont. Most digesta reach cecum and ventral colon within 3h Pellets > grass > chopped hay > hay Holding time in LI not effected by meal size vs rate of passage in SI more rapid with less frequent large meals Digestion Animal Science 126 Equine Nutrition Digestion. Lining of cecum and ventral colon have mucus secreting glands, NO enzymes All digestion due to bacteria and ciliate protozoa 7
Digestion cont. High levels of alkaline phosphatase in large intestine. Associated with high digestive and absorptive capacity This differs from dog, cat and human. Cecum and colon ~ 0.5 x 10 9 to 5 x 10 9 bacteria/g contents Very similar to rumen except less starch Digestion cont. Starch content is variable resulting in variable suppression of cellulolytic bacteria D. Frape 2010 reports as barley increases from 0 to 50% with chopped hay, digestibility of OM increases NDF and ADF decreases even though passage rate slower with barley. Digestion cont. Optimal degradation of pectin and hemicellulose at ph 5-6 Bacterial populations highest in the cecum and ventral colon. Cecal bacteria adapted to grain diets less efficient at digesting hay which can lead to impactions and visa versa which can lead to colic, laminitis, puffy swollen legs. Less efficient than ruminants at digesting hay ~80% Different bacteria Faster rates of passage 8
Digestion cont. Microbial fermentation of starch, dietary fiber and protein yields VFA s Fermentation and VFA absorption promoted by; Buffering from Na + and bicarb from ileum Anaerobic environment Normal motility to allow mixing and fermentation Acetate and butyrate products of fiber digestion Proportion of propionate and lactate increases with undigested starch ~ 7% of total glucose derived from cecal propionate (Frape D., 2010) Digestion cont. VFA absorbed into blood Water and electrolytes also absorbed Largest amount of water and electrolytes passing through ileocecal junction absorbed through cecal lumen followed by the ventral colon. Further absorption through small colon to form fecal balls. In ponies 96% of Na + and Cl - and 75% of K + and Phosphate leaving ileum are absorbed Mg and Ca absorbed mostly from SI, Phosphate from SI and LI. Net fractional absorption of P and Ca from various regions of the small and large intestine (Schryver et al 1974) 9
Digestion cont. Table 1.4 Effect of diet on the ph, production of VFAs and lactate on microbial growth in the cecum and ventral colon of the horse 7h after a meal (Frape, D., 2010) Diet ph Acetate (mmol/l) Propionate (mmol/l) Butyrate (mmol/l) Lactate (mmol/l) Total bacteria per (ml X 10-7 ) Hay 6.90 43 10 3 1 500 Concentrate plus minimal hay 6.25 54 15 5 21 800 Fasted 7.15 10 1 0.5 0.1 5 VFA s calculated as the total weight (g) of acid (as acetic acid) in the organ or as the concentration (g/100g DM) in the lumen (Elsden et al. 1946) Carbohydrate cont. Optimal VFA absorption at ph 6.5. As ph moves closer to the pk of the VFA more is absorbed H + ions required derived from mucosal cells exchanged for Na + HCO 3- buffer secreted into lumen in exchange for Cl - Net absorption of NaCl All VFA s pass intact into blood 10
Digestion cont. Lactate from stomach not well absorbed passes to hindgut where converted to propionate Microbial digestions causes gas. Mostly CO 2 methane and hydrogen. Absorbed, ejected from anus or further metabolized Negative consequences if production exceeds disposal Protein digestion. Microbial growth and dietary fiber breakdown requires available N. Dietary protein Urea secreted through lumen from blood Despite proteolytic ability of microbes protein breakdown ~ 40x s greater in ileum than in cecum and colon. Most dietary protein is absorbed different than ruminants Death/breakdown of microbes releases protein and amino acids but limited absorption 1-12% of plasma amino acid of hindgut microbial origin Protein digestion. Urea is end product of protein catabolism and excreted through kidneys. High proportion of urea produced in liver secreted into ileum. Once in cecum degraded to ammonia by bacterial urease (enzyme not in mammalian cells) Re-utilized by bacteria some goes to blood Blood ammonia kept low by healthy liver Ammonia toxicity when capacity of bacteria and liver overwhelmed 11
Intestinal gas. http://equinevetblog.blogspot.com/ http://equinevetblog.blogspot.com/ Strangulating lipoma. http://equinevetblog.blogspot.com/ Pelvic Flexure Impaction. liv.ac.uk http://equinevetblog.blogspot.com/ 12
Microbial fermentation in the stomach Animal Science 126 Equine Nutrition Microbial fermentation. Hypothesis that abnormal gastric fermentation when post-prandial DM content of stomach is high and low ph not achieved. Fermentation is normal in areas of lower ph and larger particle size As roughage particle size decreases (chopped, ground, pellet), gastric DM content decreases from 186 to 132g/kg contents and rate of passage through stomach increases. Microbial Fermentation. With large cereal meals 2.5mg/meal gastric DM increases to 400g/kg and ph 5.6-5.8 for 2-3hrs post meal. DM accumulates faster than ejected, Cereals have less chewing and saliva High DM content acts as buffer to HCl Cereals glutinous inhibiting action of gastric juices Delayed gastrin release after cereal meal Failure of postprandial ph to drop and inhibit fermentation 13
Microbial Fermentation. Lactic acid producing bacteria thrive These bacteria produce CO2 thrive at ph 5.5-6 In hour after starch meal increase in these bacteria Lactate and VFA concentrations increase If ph increases further other non-lactate forming bacteria may proliferate. Gas production greater than absorption into blood results in gastric tympany Postprandial decrease in gastric ph therefore desirable to kill potential pathogens. 14